Relaxation is a measure of the differences in the way that a molecule, such as water for example, relaxes following excitation. The relaxation rate constants of the spins located in different environments are different. A magnetic resonance image can be generated that is sensitive to the density of the water but in different tissues, the water concentration (or the density) changes little.
Relaxation provides a measure as to the environment in which the water molecule is located. For example, a water molecule excited by a radio frequency (RF) pulse gives energy off to the environment by interacting with other magnetic dipoles.
Among other factors, the rate constant depends on interaction between nearby molecules in the environment.
Longitudinal relaxation is characterized by time constant T1, e.g. time constant at which a disturbed magnetic vector returns to alignment with a static magnetic field.
Transverse relaxation, or spin-spin relaxation, is characterized by a time constant T2 at which the magnetization vector dephase in the plane perpendicular to the static magnetic field. The plane perpendicular to the static magnetic field is called the transverse plane.
Current methods for assessing relaxation and generating contrast in the rotating frame are inadequate.